As a technology for detecting an obstacle located ahead of a vehicle, Patent Document 1, for example, discloses the following technology: “When an antenna rotates in a first direction, radar signals are transmitted and received at a first group of intermittent orientation angles by the antenna to generate a first group of beat signals having frequency differences of the transmitted and received radar signals, and, when the antenna rotates in a second direction opposite to the first direction, radar signals are transmitted and received at a second group of intermittent orientation angles, which is different from the first group of intermittent orientation angles, by the antenna to generate a second group of beat signals having frequency differences of the transmitted and received radar signals. Then, a first peak shape of the first group of beat signals and a second peak shape of the second group of beat signals are detected, and on the basis of a maximum value in the first peak shape and a maximum value in the second peak shape, the angle to a target is detected. It is, therefore, possible to avoid a reduction in the accuracy of detection of the angle to the target. (Extracted from the Abstract)”.